Methods and system for processing of drilling fluid

ABSTRACT

A method and a system are described for processing of drilling fluid from a drilling hole ( 10 ) in an underwater well to a floating drilling rig or drilling vessel, comprising a sealing device ( 12 ) connected to a well head, a pump module ( 14 ) to transport drilling fluid, a treatment plant for treatment of drilling fluid, or a storage installation. Before a blow-out valve is connected to the well head and before a riser is connected between the drilling hole and the floating drilling rig or drilling vessel, the pump module ( 14 ) placed on the ocean bed transports drilling fluid from the drilling hold ( 10 ) on the ocean bed to the floating drilling rig or drilling vessel.

The present invention relates to a method and a system for processing ofdrilling fluid from a drilling hole in an underwater well to a floatingdrilling rig or drilling vessel. In particular, the invention relates toprocessing of drilling fluid before a blow-out valve is connected to thedrilling hole and a riser is connected between the drilling hole and thefloating drilling rig or drilling vessel.

Today's demands relating to environmental discharges puts great demandson the operators in the oil industry. For example, some of the operatorsstipulate that there shall not be any discharge of drilling fluid duringdrilling. During drilling of a new oil well in the ocean bed, ordrilling in an already existing well, large amounts of drilling fluid,which must be treated, are produced. This car be oil-based drillingfluid or water-based drilling fluid, depending on whether the drillingwhich is being carried out, is top-hole drilling or drilling in the coilzones.

In this application, drilling fluid is meant to be fluids which appearduring drilling in a drilling hole, such as cuttings, drilling mud, orother drilling fluids.

In recent years, the environmental threats which the oil industry poseshave been given increasingly more focus. The authorities have imposedincreasingly stronger demands on care for the environment and havestrict rules for discharges from offshore installations, as these canhave negative effects on the maritime environment. Today, there are, inthe main, strict restrictions with regard to discharges of oil-baseddrilling mud, and discharges of this type have almost been completelystopped in the Norwegian sector of the North Sea.

In a standard well, in which the following holes are drilled withoutrisers (36″-225 m, 26″-1200 m), more than 340 m² of cuttings will beproduced directly from the well. In addition, there is the drilling mudwith its mixture of different chemicals. The Norwegian Pollution ControlAuthority (SFT) introduced a complete ban on dumping of drilling mudand/or drilling fluid in the Norwegian sector of the North Sea in 1993.This was the start of what is today called slurry-fixing plants, whichare able to process the return of fluid to the drilling hole.

Today, most of the fixed installations have such plants, but they areonly used for injection of oil-containing waste. The injection iscarried out in an annular space between two casings in the drillinghole, normally casings with diameters of around 340 mm and 508 mm (13⅜″and 20″). This is based on a pump rate of about 4000 1/min underdrilling of about a 311 mm (12½″) section and about a 216 mm (8½″)section.

Water-based drilling fluids are discharged directly to the sea and sinkto the ocean bottom, something that creates environmental problems forthe maritime life both in the ocean and at the ocean bottom. Dischargesof drilling fluids can be carried out with the aid of a pump which isconnected on a base at the drilling hole The pump acts as a suction pumpto create a negative pressure in a sealing device which is arrangedround the drill column in the drilling hole.

Disadvantages with today's methods are that if the water-based drillingfluid is to be transported up to the drilling rig to be injected into acorresponding well, many problems to which there are no solutions atpresent arise. For example, during top-hole drilling, there are nomaritime risers, i.e. a vertical riser which transports drilling mudfrom the ocean bottom and up to the drilling platform, and in addition,there is no annular space for injection of the water-based drillingfluid.

U.S. Pat. No. 4,149,603 disclose a system and a method of underwaterdrilling operation, which returns drilling mud to the surface of thewater, without the use of a riser, but after a BOP is installed. Thesystem comprises e mud sump connected to the top of a submerged wellheadand pump means to pump mud through a hose and to the surface.

EP 0290250 discloses a method and apparatus for drilling sub sea wellsat large depths, where drilling return mud is pumped to the surface by acentrifugal pump. The apparatus is attached to top of the blow-outpreventer stack.

None of the prior art documents discloses methods or apparatuses adaptedto be used before a riser is connected and a blow-out preventer isinstalled on the wellhead.

There is, therefore, a need for a method that can remove discharges ofdrilling fluid returns at a drilling rig or drilling vessel, and whichcan be applied in connection with the already existing drilling holeapplications both on the ocean bottom and on the drilling rig, beforeboth riser and blow-out preventer is installed. There is also a need fora system to carry out the method according to the present invention.

Advantages with the method according to the present invention are thatgreat savings are achieved by being able to recirculate drilling fluidreturns. Full drilling rate is maintained in the different sections,i.e. about 311 mm and about 216 mm (12¾″ and 8½″) sections. Moreover,the environment is spared from unnecessary discharges. A fasterslurrification of the drilling fluid which is produced during drillingis also achieved, i.e. faster treatment of the pumpable fluid or mudwhich consists of a solid material sedimented in a fluid. Less strictdemands for the slurry. No wearing of casings will occur, and there isno danger that the casing will be damaged. Drilling fluid is kept awayfrom the template, i.e. the base, and no concrete is used around thetemplate. This gives a clear view for the ROV operator (RemotelyOperated Vehicle). A greater injection rate is also achieved. Inaddition, the drilling fluid can also be stored for later, to betransported away from the floating drilling rig.

In connection with drilling on the ocean bed, drilling fluid is formedaround the drilling mould (template). It is normal to use remotecontrolled underwater vehicles (ROV-“remote operated vehicle”) with acamera, to monitor and carry out various operations, and the drillingfluid/mud in the area around the drilling hole orifice represents,therefore, a considerable visual problem. Cuttings are fragments ofrocks, which under drilling are brought up with the drilling mud.

The object of the prevent invention is, therefore, to provide a methodand a system, which eliminates the abovementioned problems. It is alsoan object to provide a method and a system of processing drilling fluidreturn from a drilling hole in an underwater well at a floating drillingrig or a drilling vessel, comprising a sealing device connected to awell head, and a pump module to transport drilling fluid, a treatmentplant, or a storage installation, for drilling fluid and possibly aninjection pump.

The method, according to the present invention, is characterised in thatbefore a blow-out valve is connected to the well head, the submergedpump module and the sealing device provides an outlet pressure,dependent on the specific weight of the mud and the ocean depth, whichis high enough for transportation of the drilling fluid from thedrilling hole, through the return line and up to the floating drillingrig or drilling vessel.

The system, according to the present invention, is characterised in thata pump module, which is arranged on the ocean bed and connected to asealing device, is adapted to transport drilling fluid from the drillinghole on the ocean bed, via a return line, to a treatment plant, or astorage installation, on the floating drilling rig or drilling vessel.

Preferred embodiments of the method, according to the present invention,are specified in that the pump nodule placed on the ocean bed and thesealing device provides an outlet pressure, dependent on the weight ofthe mud and ocean depth, which is high enough to transport drillingfluid from the drilling hole, through the return line and up to thefloating drilling rig or drilling vessel. The drilling fluid istransported through the return line and to the existing line (flow-line)on the floating drilling rig or drilling vessel for further transport tothe treatment plant or storage installation. After the cuttings istreated, using a method that may be known previously, on the floatingdrilling rig or drilling vessel, the treated cuttings is injected, withthe aid of a high-pressure pump, into a second drilling hole provided onthe ocean bed, or in an adapted annular space in the first drillinghole.

Preferred embodiments of the system are characterised in that thesubmerged pump module and the sealing device, before a blow-out valve isconnected to the well head, are adapted to provide an outlet pressurewhich is high enough for transportation of the drilling fluid from thedrilling hole, through the return line and up to the floating drillingrig or drilling vessel.

The pump module placed on the ocean bed comprises a number of pumps toprovide the necessary pressure, such as a centrifuge and/or a frictionpump possibly connected in series, where the pump, or pumps, is drivenby a submerged electric motor which is connected to the pump or pumps.

A preferred embodiment, according to the present invention, shall now bedescribed with reference to the enclosed figures. It must be understoodthat this example is not limiting and that other and furthermodifications may be carried out within the scope of the claims.

FIG. 1 shows a principle of the method and the system for processing ofa drilling fluid according to the present invention.

FIG. 2 shows a section of an injection well according to FIG. 1.

To a first drilling hole 10 which is already drilled in the ocean bed,it is common to connect a sealing device 12, which normally is describedas a suction and centralisation module (SCM), as shown in FIG. 1. Thissealing device 12 is connected to the well head of the first drillinghole 10, for example, to form a seal between the foundation at the wellhead and a pipe string up to the drilling rig, and to create a negativepressure in the drilling hole for suction of drilling fluid.

The present invention applies, amongst other things, such a knownsystem, with a sealing device for removal of drilling fluid from adrilling hole orifice, which is characterised in that between the innersurface of the casing and outer surface of the drilling column anend-piece which forms a seal, basically a watertight seal, is arrangedbetween the casing and the drilling column, and that at least one exitpassage is arranged in the casing which is connected directly to a linesystem whereupon a pump module, for example, can be connected. Thissystem is based on the applicant's Norwegian patent application no.19982394.

A pump module 14 is connected to this exit passage or outlet on thesealing device 12 for suction of drilling fluid/drilling mud. The outletpressure is dependent on weight of mud and water depth. For example, ata water depth of 400 m and a mud weight of 1.7, the pressure will beapproximately 22 bars, Because of the negative pressure in the well head10 generated by the sealing device 12 and the pump module 14, a liftingheight, including pressure drop and lifting reduction because of theweight of the slurry, is generated, sufficient to lift the drillingfluid up to an existing line on the drilling rig, for example an alreadyexisting “flow line”, which is well known to those skilled in the art.Transport of the drilling fluid from the pump module 14 to the existingline can, for example, be carried out in a about 152 mm (6″) or 203 mm(8″) pipe/line 16 which is connected to the already existing line (flowline) on the drilling rig. The pipe 16 must be of a type which canwithstand the working-pressure which is necessary to lift the slurry upto the floating drilling rig or drilling vessel.

The pump module comprises a pump of known type which can pump seawater,drilling fluid and cuttings under high pressure. At greater depths, itmay be necessary with a multi-step solution, for example, two or morepumps connected in series, to obtain the required pressure The pump isarranged as a module which can be tested and thereafter lowered down tothe ocean bed ready for use after pipes have been connected to the inletand outlet. To reduce the weight and dimensions, it seems sensible touse a centrifugal and/or friction pump driven by a submerged electricmotor, which is connected directly to the pump. The power supply can bearranged in a compounded umbilical cord (umbilical), which can also beused to lower the pump down to she ocean bed.

After transport of the drilling fluid to the floating drilling platformor drilling vessel, the drilling fluid is thereafter led to a treatmentplant, or alternatively, a storage installation on the floating drillingrig or drilling vessel for further transport to another treatment plantor storage installation.

The treatment plant on the floating drilling rig or drilling vesselcomprises, for example, a shaking unit shaker), a first storage tank, amixing tank, a crushing unit, other storage tanks, and a high-pressureinjection pump, etc.

The water-based drilling mud is strained in the shaking unit. Extraseawater is strained and returned to a storage tank, for mixing ofslurry for injection. When this method is used, approximately 80 to 90%of the water-based drilling mud can be recirculated. This gives verylarge cost savings per day during, for example, top-hole drilling. Afterthe drilling fluid has been strained in the shaking unit, it istransported to a tank which comprises a number of crushing units orcrushing pumps. The slurry is crushed in the crushing units or crushingpumps to a preferred particle size of around 10 to 20 μ, or anothersuitable size, whereupon the matter is pumped to a storage tank beforeit is transferred to an injection unit, such as for example ahigh-pressure pump, for injection into a second drilling hole 18. Thisinjection can, for example, be carried out in a 102 mm (4″) injectiontube 20 with a working pressure of between approximately 35-150 bars.

The methods according to the present invention, can also include that aninjection well is drilled at a distance from the first drilling hole 10.An example of a new injection well is shown in FIG. 2, and can, forexample, be a well 18 which is drilled for placing of a 178 mm. (7″)casing 22 in a 340 mm (13⅜″) casing 24, with, for example, a well depthof approximately 500 to 1500 m. This well depth can also vary, dependingon the formation which is being drilled, and how receptive the formationis to the drilling fluid which is to be injected. An area 26 of thelower part of the inner casing is perforated for injection of thewater-based drilling fluid.

Injection of the drilling fluid can also be performed in the firstdrilling hole (10), in a suitable annular space which may be between thecasing and formation.

The drilling fluid, which is stored in the storage tank on the drillingrig, is injected with by the high-pressure pump, and through a wellheadsystem which is connected onto he well. This wellhead system can be of atype which, for example, gives a wear-free injection and which alsoincrease the capacity of the injection.

In principle, the treatment plant can be placed at an arbitrary place aslong as the drilling fluid can be pumped to the treatment plant and thedrilling fluid can be injected into the second drilling hole. In thefirst example conducted, the treatment plant is placed on the drillingrig, because the already existing treatment plant is normally installedthere, but the treatment plant for the drilling fluid can, off course,be placed somewhere else.

Thus, a new method and system for transport drilling fluid from adrilling hole on the ocean bed to a floating drilling rig or drillingvessel is provided, improving the environment in the sea.

What is claimed is:
 1. A system for processing drilling fluid duringtop-hole drilling in underwater drilling operations comprising: asealing device for mounting over a drilling hole in sealed relation tothe surroundings seawater to prevent leakage of drilling fluid from thedrilling hole; a floating drilling vessel having at least one of atreatment plant for treating drilling fluid and a storage installationto receive drilling fluid; at least one pump module spaced from andconnected to said sealing device to effect a differential pressuretherein for pumping drilling fluid from said sealing device upwardly tosaid at least one of said treatment plant and said storage installationon said vessel; and a line extending from said pump module upwardly tosaid at least one of a treatment plant and a storage installation onsaid vessel to convey the drilling fluid from said pump module to saidat least one of a treatment plant and a storage installation on saidvessel.
 2. A system as set forth in claim 1 wherein said sealing deviceand said pump module are interconnected to form a suction andcentralizaton module.
 3. A system as set forth in claim 1 furthercomprising a submerged electric motor operatively connected to said pumpto drive said pump.
 4. A system as set forth in claim 1 wherein saidpump generates an outlet pressure dependent on the ocean depth andweight of the drilling fluid sufficient to transport the drilling fluidto said drilling vessel.
 5. A method of processing drilling fluid from adrilling hole in an ocean bed during top-hole drilling before a blowoutpreventer is installed and a riser connected between the blowoutpreventer and a drilling vessel, said method comprising the steps of:mounting a sealing device over the drilling hole in sealed relation tothe surroundings seawater; mounting at least one pump module in spacedrelation to and connected to said sealing device to effect adifferential pressure therein; providing an outlet pressure for thedrilling fluid based on said differential pressure and the specificweight of mud to be transported and the ocean depth; and pumpingdrilling fluid from the sealing device into a line extending upwardly toat least one of a treatment plant and a storage installation on a saidfloating drilling vessel.
 6. A method as set forth in claim 5 whichfurther comprises the step of returning the drilling fluid from thedrilling vessel into the drilling hole.
 7. A method as set forth inclaim 5 which further comprises the step of directing the drilling fluidfrom the drilling vessel into the ocean bed in spaced relation to thedrilling hole for injection into a further drilling hole.